Happy to share, we have implemented split button and remember last action for Font color, Fill color, and Border.
You are welcome to check it out and provide us feedback through provide feedback in the app.

Happy to share, Excel Online now supporting the ability to Select All cells in a range, a table, or sheet; by using Ctrl+A or by clicking the triangle at the top left corner. You are welcome to try it and provide us feedback through the service (File → Help → Provide feedback to Microsoft)

Given the differences between excel online and the desktop app, i would suggest that things such as Find next be a bit more visible as options, or at least make a link to the keyboard shortcuts more visible.

In the desktop Excel app, users can use Command + Shift + up/down arrow keys to quickly select a set of cells without having to manually drag the mouse to select all of them. Please bring this keyboard shortcut to the Excel Online app as well!

Great news – we’ve made much of the functionality that was previously available only in the hosted Office Online offering available on-prem. Data Validation, Sheet Protection, etc. are available there now.

I am happy to announce that shapes can now be viewed! 
Enabling shapes for editing and insert shapes is on our backlog.
Thank you all for voting and influencing Excel Online priorities.
Keep voting for other ideas you want the product team to address.

Word and PowerPoint online have embedding options that generates an embed code for you to embed documents direction into other websites. It would be great to have that for Excel as well. While you can manually create a working embed code, it'd be much easier as an option just like with Word and PowerPoint!

You can already edit spreadsheets in the browser. All you need to do is upload your file to OneDrive.com (or your O365 account); make sure you are signed in to the browser with your Microsoft Account (@hotmail.com, @outlook.com, etc.) and then click on the “Edit” button and choose “Edit with Excel Online”